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Abstract
We have developed and experimentally demonstrated a highly coherent and low noise InP-based InAs quantum dash (QDash) buried heterostructure (BH) C-band passively mode-locked laser (MLL) with a pulse repetition rate of 25 GHz for fiber-wireless integrated fronthaul 5G new radio (NR) systems. The device features a broadband spectrum providing over 46 equally spaced highly coherent and low noise optical channels with an optical phase noise and integrated relative intensity noise (RIN) over a frequency range of 10 MHz to 20 GHz for each individual channel typically less than 466.5 kHz and -130 dB/Hz, respectively, and an average total output power of ∼50 mW per facet. Moreover, the device exhibits low RF phase noise with measured RF beat-note linewidth down to 3 kHz and estimated timing jitter between any two adjacent channels of 5.5 fs. By using this QDash BH MLL device, we have successfully demonstrated broadband optical heterodyne based radio-over-fiber (RoF) fronthaul wireless links at 5G NR in the underutilized spectrum of around 25 GHz with a total bit rate of 16-Gb/s. The device performance is experimentally evaluated in an end-to-end fiber-wireless system in real-time in terms of error vector magnitude (EVM) and bit error rate (BER) by generating, transmitting and detecting 4-Gbaud 16-QAM RF signals over 0.5-m to 2-m free-space indoor wireless channel through a total length of 25.22 km standard single mode fiber (SSMF) with EVM and BER under 8.4% and 2.9 × 10−5, respectively. The intrinsic characteristics of the device in conjunction with its system transmission performance indicate that QDash BH MLLs can be readily used in fiber-wireless integrated systems of 5G and beyond wireless communication networks.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
To meet the ever-increasing demand for high bandwidth, high speed, low latency, and to overcome the global shortage of RF spectrum, frequency bands above 24 GHz in the millimeter-wave (mm-wave) spectrum with plentiful available bandwidths are envisioned for 5G wireless networks [1]. This paradigm shift in the RF spectrum for wireless carriers is inevitable in order to support the bandwidth hungry services and applications, such as Gb/s wireless connectivity, ultra-high definition video streaming, Internet of Things (IoTs), smart home/building, virtual and augmented reality (VR/AR), autonomous vehicles, and work and play in the cloud. Enhanced mobile broadband (eMBB) is one of the three 5G standard usage scenarios set by the International Telecommunication Union (ITU) in the International Mobile Telecommunications 2020 (IMT-2020) that features peak data download speed of more than 20-Gb/s and seamless 100-Mb/s user experience data rates in wider coverage area with expected speed of Gb/s in hotspot cases [2]. Such ultra-high speed and broadband wireless signals cannot be realized and supported by the already depleted sub-6 GHz RF spectrum. Therefore, to fulfill the requirements of IMT-2020, 3GPP has identified and standardized the underutilized high frequency bands (24.25 GHz to 52.6 GHz) in the mm-wave spectrum for the development of 5G NR (a global standard for 5G wireless interface) systems [3]. Nevertheless, to generate, process, control and distribute such ultra-high speed and wideband wireless signals in the electrical domain may not be feasible due to the electronics bottleneck [4–6]. Microwave photonics with RoF is identified one of the key enabling technologies for generation and distribution of high bandwidth and ultra-high speed mm-wave RF signals in the optical domain [4–7]. Moreover, in order to satisfy the 5G requirements, fiber-wireless integration is indispensable. In addition to being immune to electromagnetic interference, optical fiber offers high capacity, low latency and large transmission distance but limited mobility. On the other hand, wireless networks provide high mobility and seamless coverage but limited bandwidth and transmission distance. Therefore, seamless fiber-wireless convergence is necessary for ubiquitous and multi-gigabit wireless connectivity [6]. The simplest approach to generate wideband high speed RF wireless signals and to achieve fiber-wireless convergence is the photonic-aided frequency up-conversion through remote heterodyne mixing of two coherent optical signals running over several tens of kilometer of optical fiber, with the modulated data onto either one or both of them and spaced at the desired mm-wave RF carrier signal, on a high speed photodetector [8,9]. This not only overcomes the problem of electronics bottleneck, transmission span limitation and provides seamless fiber-wireless conversion, but also greatly reduces system and network complexity, foot-print, as well as Capital Expenditures (CAPEX) and Operating Expenses (OPEX) considering the expected deployment of ultra-dense small cells with large number of 5G remote radio units (RRUs) having massive multiple-input-multiple-output (MIMO) antennas connected to the baseband units (BBUs) in a centralized cloud environment through an optical fiber-based fronthaul network [10].
Numerous techniques have been developed and demonstrated under the umbrella of microwave photonics for mm-wave RF signals generation over optical links, ranging from optical heterodyne beating of two single wavelength individual laser sources [11] and dual-wavelength lasers [8,12,13] to employing optical modulator(s) [14–17] and optical frequency combs based on various schemes [4,18–30]. Among them, optical coherent frequency combs (CFCs) based on integrated monolithic semiconductor quantum dash (QDash) or quantum dot (QD) passively mode-locked lasers (MLL) [25–29] are very attractive due to simple and compact design and broadband flat spectra having multiple highly coherent and low noise optical channels. This promises a low cost and flexible solution for 5G and beyond wireless networks, particularly in the application of fronthaul with massive MIMO RRUs. Because a single CFC can be used to not only replace many individual laser sources in the central office (CO), but also provide common coherent optical local oscillators (LOs) for photonic up-conversion of RF signals in the RRUs that greatly simplifies the overall system and network complexity. In addition, the noise performance (phase noise and RIN) of the optical sources directly affect the frequency instability and phase noise of the photonically generated RF carrier signals, which in turn impairs the performance of RoF wireless transmission systems [21,31,32]. Therefore, highly coherent low noise optical sources are considered instrumental in facilitating future photonic mm-wave RF wireless communication systems. Owing to their inherent superior characteristics, such as reduced amplified spontaneous emission, large effective gain bandwidth, ultra-fast carrier dynamics, and improved temperature stability [33], QDash/QD materials based lasers are capable of achieving stable mode-locking with multi-wavelengths having very narrow optical spectral linewidths and low RIN as compared to quantum-well based lasers (QWs) [12,34–38], which are currently being used in most commercially available laser sources. Therefore, QDash/QD based semiconductor MLLs with high spectral purity, compact size, low cost, low power consumption, simple fabrication and integration feasibility have great potential as efficient chip-scale candidates for deployment in fiber-wireless integrated 5G NR RoF-based fronthaul systems.
InAs QDash lasers grown on InP substrates have been studied for many years [39], with many mode-locked devices operating in the C-band [35,38,40–46] and L-band [46–52] being reported. These include devices based on single section [35,38,40–46,48,50,52] and two-section [43,47,49,51] configurations, with the single-section self-pulsating MLL devices being considered advantageous due to better pulse generation with increase average output power [53]. Recent demonstrations show the potential of these InP based QDash MLLs for mm-wave applications. These include the demonstration of RF mm-wave signal generation [54,55] and wireless transmission [29] using injection-locked L-band QDash lasers, and Gb/s RoF wireless communication systems with [24,56,57] and without RF wireless links [25,58] using injection-locked C-band QDash MLLs. However, most of the reported QDash devices in the literature demonstrate relatively broad free running spectral linewidths. It is believed that the buried heterostructure (BH) configuration with blocking layers has superior characteristics compared to the typical ridge configuration in terms of linewidth, frequency noise, threshold current, and efficiency. BH structures also improve the mode symmetry with lower divergence angle resulting in better coupling to optical fibers. Nevertheless, BH QDash MLL devices and their application in end-to-end RoF systems with RF wireless links have not yet been fully explored. Consequently, it is worth investigating to fully exploit the superior characteristics of BH QDash MLLs for RoF systems of 5G and beyond wireless communication networks.
Over the past decade, we have reported different InAs/InP QDash/QD Fabry-Perot (F-P) MLLs in the C- and L-bands with channel spacing that can cover an RF frequency range from tens of GHz to THz [35,38,44,59–62]. In this paper, we report a 25 GHz QDash BH MLL, its design, fabrication, experimental characterization and demonstration in an end-to-end proof-of-concept RoF system with RF wireless links. Broadband photonics-assisted wireless links at 5G NR with data throughputs of 16-Gb/s (4Gbaud 16-QAM) over 25.22 km SSMF and 0.5-m to 2-m free-space wireless transmission distance are experimentally demonstrated employing the QDash BH MLL with EVM well below the standard limit of 12.5% [3] and BER under the 7% overhead forward error correction (FEC) requirement of 3.8 × 10−3.
2. Laser design, fabrication and experimental characterization
The device presented in this paper is an InP-based p-n blocked buried heterostructure Fabry-Perot (FP) laser. Figure 1(a) shows a schematic of the cross-section of the laser. The laser structure is comprised of a 170 nm thick InGaAsP waveguide core with 10 nm In0.816Ga0.184As0.392P0.608 (1.15Q) barriers embedding five stacked layers of InAs QDashes as the active gain region surrounded by n- and p- type InP cladding layers. The 10 nm barriers are thick enough to avoid electronic coupling between QDash layers, but correlated vertical stacking of the dashes is observed due to the strain fields. The lower n-type InP cladding contains 1.03Q ballast layers to pull the optical mode into the n-type region. This reduces cavity loss, increases efficiency, and reduces the coupling of spontaneous emission to the optical mode. Chemical beam epitaxy (CBE) was used to grow the InAs QDash material in a manner similar to that in [40], with the further growth steps required for creating a buried heterostructure being performed by MOCVD. The average QDash density in each active layer was around 3.5 × 1010cm−2. A scanning electron microscope (SEM) image of a typical QDash layer used in the fabrication is shown in Fig. 1(b). The 1735 µm long laser waveguide was fabricated via standard photolithography in combination of dry-, wet-etching and contact metallization techniques. After growing the laser core, a 2 um wide waveguide mesa was created by etching through the 1.15Q waveguide core followed by the selective overgrowing of pnp blocking layer structure to confine current to the waveguide mesa. Then after removing the selective area dielectric mask, the final p-type InP cladding and contact layers were grown. Both facets of the laser were left uncoated. A SEM image of the front cross-section of a fully fabricated QDash BH MLL laser is shown in Fig. 1(c).
Fig. 1. (a) Schematic of the cross-section of the laser. SEM images of (b) the top view of the quantum dashes layer and (c) front cross-section of the fully processed QDash BH MLL.
Furthermore, the 1735 µm long laser chip is mounted on a commercially available Aluminum Nitride (AlN) carrier with two gold (Au) electroplated contacts to provide mechanical support and electrical connection to the laser chip, respectively. The bottom contact of the QDash BH MLL chip-on-carrier (CoC) provides a cathode connection through Eutectic Gold Tin (AuSn), whereas the top contact through wire-bonding provides the corresponding anode connection. For the experimental characterization of the QDash BH MLL, the CoC is placed on a copper block with a thermoelectric cooler (TEC) underneath. A laser diode controller (LDC) (ILX Lightwave, Model LDC-3722) is used to DC bias the CoC through a pair of probes and drive the corresponding TEC to control its temperature. The laser output light is collected from its front facet using a collimated lensed polarization maintaining (PM) fiber attached to an isolator. The position of this fiber is adjustable in three dimensions for coupling the light from the laser cavity. The free running passively MLL without any controlled feedback is characterized by measuring its light – current (L-I) curve, optical spectrum, phase noise, RIN and RF beat note.
Figure 2(a) shows a typical L-I characteristic curve of the QDash BH MLL measured at 19 °C. It can be seen that the laser starts lasing at around 50 mA and provides a maximum average output power of ∼50 mW. The laser output spectrum is characterized using an optical spectrum analyzer (Anritsu, Model AQ6317B) with a spectral resolution of 0.01 nm. Figure 2(b) shows the optical spectrum of the laser at 423 mA. The laser operates in the shorter part of the C band with its central wavelength of around 1531 nm and 6-dB optical bandwidth of around 9 nm providing ∼ 47 highly coherent optical channels with an optical signal-to-noise ratio of better than 40 dB as shown in Fig. 2(b). Moreover, to characterize the noise performance of the device, selected individual optical channels covering the spectrum of the QDash BH MLL in the C-band are filtered out using an optical band pass filter (EXFO XTM-50-SCL-S-M) for optical phase noise and RIN analysis. The optical frequency noise of the selected individual channels is measured using an automated laser linewidth/phase noise measurement system (OE4000 OEWaves Inc.). The corresponding optical spectral linewidth is estimated from the measured optical frequency noise spectrum of the selected channels, which is between 306.2 kHz to 133.6 kHz as shown in Fig. 3(a). Similarly, the RIN is measured using a RIN measurement system (Agilent N4371A) and the integrated RIN for the selected individual channels over a frequency range of 10 MHz to 20 GHz is calculated between -130.7 dB/Hz and -136.6 dB/Hz as shown in Fig. 3(b). It is observed that the QDash BH MLL achieves RIN and optical linewidth for each individual optical channel typically less than -130 dB/Hz and 466.5 kHz, respectively. Furthermore, a 50 GHz signal analyzer (Keysight, Model N9030A) with a high speed IR photodetector (New Focus, Model 1014) is used to investigate the RF phase noise and timing jitter performance of the QDash BH MLL between any two of its adjacent optical channels. It is observed that the device exhibits RF 3-dB linewidth down to 3 kHz and timing jitter as low as 5.53 fs. The 3-dB RF linewidth is estimated from measured RF spectrum of the beat-note signal at around 25.89 GHz using Lorentzian fit as shown in the inset of Fig. 3(a). The corresponding timing jitter is calculated from the obtained 3-dB RF linewidth [41]. Besides the simple design and fabrication as well as good optical noise performance of the laser, this narrow beat-note linewidth and low timing jitter not only shows high coherence between the optical channels of the laser but are of paramount importance for the spectrally pure mm-wave RF signals generation and data transmission in 5G optical heterodyne RoF systems.
Fig. 3. Measured (a) optical spectral linewidth for individual selected channels (inset showing RF beat-note of around 25.089 between any two adjacent channels with 3-dB linewidth ≤ 3 kHz measured at the resolution bandwidth of 5.1 kHz and video bandwidth of 100 Hz), and (b) integrated RIN for individual selected channels over a frequency range of 10 MHz to 20 GHz.
3. System experimental setup and results
The performance of QDash BH MLL is evaluated in a system experiment by realizing photonics-assisted RoF wireless data transmission links. The experimental setup with the corresponding signal’s spectra at different points along the transmission path is shown in Fig. 4. In our experimental demonstration, we emulate a typical 5G fronthaul scenario with centralized radio access network where remote optical heterodyne of two optical channels (data channel and optical LO) is used to up-convert the baseband data signals to the desired RF carrier signal in optical domain at the RRU without using any RF electrical LO.
Fig. 4. Schematic of the experimental setup for photonics-assisted RoF RF signal generation, data transmission and detection using QDash BH MLL with the insets showing spectra (i) of the selected two optical channels (data channel and optical LO) at the CO before modulation, and (ii) after 25.22 km SSMF transmission (data channel modulated) at the RRU before the FOR.
In the CO, the QDash BH MLL device is used as a main optical source, which is biased at 422.78 mA and maintained at 19 °C using the LDC. The output of QDash BH MLL is connected to a 50/50 PM optical coupler (OC1) through a two stage PM isolator, to avoid any back reflections, followed by two tunable optical band pass filters (OBPF1 and OBPF2). The two OBPFs are used to select two adjacent optical channels; one for data transmission and another as an optical LO for photonic up-conversion at the RRU, spacing at the RF carrier frequency near to the center of the standard 3GPP 5G NR frequency band n258 (24.25 to 27.5 GHz). The inset (i) in Fig. 4 shows the optical spectra of the two selected optical channels at 1533.044 nm (λ1) and 1533.240 nm (λ2), respectively, that are filtered out from the free running optical comb spectrum obtained from the QDash BH MLL cavity without employing any controlled feedback mechanism. It is noteworthy that the selection of the optical channels is flexible and the QDash BH MLL can be used to generate RF signals in higher frequency bands of mm-wave spectrum including K-band, V-band, W-band and even THz range depending on the channels spacing of the selected tones. The measured optical frequency noise and RIN spectra for the two selected optical channels are shown in Fig. 5(a) and Fig. 5(b), respectively. For these frequency noise spectra, the estimated optical spectral linewidths for λ1 and λ2 are 167.878 kHz and 141.828 kHz, respectively. Their integrated RIN estimated from the RIN spectra over a frequency range of 10 MHz to 20 GHz is -134.384 dB/Hz and -136.655 dB/Hz, respectively. After boosting the power of optical data channel (λ1) by an erbium-doped fiber amplifier (EDFA1), it is modulated with 16-QAM digital baseband signals having a symbol rate of 4-GBaud by employing two channels (in-phase (I) and quadrature (Q)) of a dual polarization (DP) QAM optical transmitter system (SHF 46215B DP-QAM). The 4- GBaud 16-QAM baseband signals are generated electronically using an arbitrary waveform generator (AWG) (Keysight Technologies, Model M9502A) with a pseudo random binary sequence (PRBS) pattern of 2−11-1 bits and a root raised cosine (RRC) filter with roll-off factor of 0.35 is also employed for pulse shaping. The second optical tone (λ2) is used as a supplementary channel to provide optical LO for heterodyne RF carrier signal generation at the RRU. The modulated and unmodulated optical signals are then combined in a 50/50 OC2 and transmitted over a 25.22 km SSMF to the RRU.
Fig. 5. Measured (a) optical frequency noise and (b) RIN spectra for the selected modulated (λ1) and unmodulated (λ2) channels (data channel and optical LO) of the QDash BH MLL.
In the RRU, the received optical signal is amplified by an EDFA2 followed by OBPF3 to filter out the effect of amplified spontaneous emission (ASE) noise and other optical components. The corresponding optical spectrum of the received 4-Gbaud modulated (λ1) and unmodulated (λ2) signals (data channel and optical LO) at the RRU is shown in the inset (ii) of Fig. 4. These two optical signals are then beat together on a 38-GHz bandwidth fiber-optic receiver (FOR) (New Focus model 1474-A) to generate the 16 Gb/s (4-GBaud × 16-QAM) modulated RF carrier signal at ∼25.09 GHz. The FOR is directly attached to a 17 dBi Horn antenna (WR-34) with a 20-33 GHz bandwidth that transmits the generated 4-GBaud RF data signal over 0.5-m to 2-m free-space indoor wireless distance. The wireless link of up to 2-m is limited by the congested space in our Lab. The RF signal is then received by another identical Horn antenna and amplified by a low noise amplifier (LNA) before capturing into a real-time oscilloscope (RTO) for processing. The signal is captured and coherently detected in real-time by using a 33-GHz bandwidth and 100 GSa/s speed Tektronix DPO73304SX oscilloscope with vector signal analysis software (SignalVu). This process involves several digital signal processing (DSP) steps before the signal is demodulated, which include signal down conversion, carrier and symbol locking, RRC matched filtering to recover the baseband IQ data and to minimize intersymbol interference (ISI), adaptive equalization to compensate for linear distortions, symbols detection and data decoding to calculate the EVM and BER. Finally, the system performance is evaluated by analyzing EVM and BER of the received decoded 16-QAM 4-GBaud (16-Gb/s) data signals. The BER is obtained from the EVM measurements [63].
The QDash BH MLL performance is evaluated in a proof-of-concept end-to-end fiber-wireless integrated system experiment by generating, transmitting and detecting broadband RoF wireless signals in the downlink in real-time in terms of EVM and BER. In the experiments, photonics-assisted frequency up-conversion and detection of 4-GBaud 16-QAM baseband data signals is successfully demonstrated over 25.22 km SSMF and 0.5-m to 2-m free-space indoor wireless RF links with EVM and BER below the standard limit of 12.5% and FEC requirement of 3.8 × 10−3, respectively. Figure 6(a) shows the estimated BER as a function of RF link distance at a fixed received optical power (ROP) of around 2 dBm. It can be seen that for all the RoF wireless links, a BER of well below the standard FEC is achieved. Moreover, Fig. 6(b) shows the corresponding constellation diagrams of the received 16-QAM data signals with the measured rms EVM of 6.62%, 6.92%, 7.72%, and 8.43% for RF wireless link distance of 0.5-m, 1-m, 1.5-m, and 2-m, respectively. In addition, the clear and open eye diagrams for the corresponding 4-Gbaud received signals are shown in Fig. 6(c). These results indicate that low noise monolithic integrated InAs/InP QDash BH MLL with broadband flat spectra having multiple highly coherent optical channels is a promising optical source for potential applications in high speed and high capacity 5G fiber-wireless integrated systems.
Fig. 6. Experimental transmission performance of the QDash BH MLL in photonics-assisted 16-Gb/s (4-Gbaud × 16 QAM) wireless links at ∼25.09 GHz 5G NR over 0.5-m to 2-m RF link distance through 25.22 km SSMF (a) obtained BER, (b) 16-QAM constellations with measured EVM at different wireless distances, and (b) their corresponding eye diagrams observed for 4-Gbaud 16-QAM signals after (i) 0.5-m, (ii) 1-m, (iii) 1.5-m, and (iv) 2-m, wireless links.
4. Conclusion
We have developed, experimentally characterized and demonstrated a 25 GHz monolithically integrated InAs/InP quantum dash buried heterostructure mode-locked laser (QDash BH MLL) for broadband fiber-wireless integrated RoF-based 5G NR systems. The QDash BH MLL has a compact design featuring low threshold current, high output power and flat broadband spectrum with 6-dB bandwidth of around 9 nm having ∼47 equally spaced highly coherent and low noise optical channels. The noise performance of each filtered individual channel of the developed 25-GHz QDash BH MML is experimentally characterized with an optical phase noise of less than 466.5 kHz and an integrated average RIN of typically less than -130 dB/Hz over the frequency range from 10 MHz to 20 GHz. Its RF beat note linewidth between any two adjacent channels is measured to be 3 kHz with a calculated time jitter of 5.53 fs. By using this QDash BH MLL, we have successfully demonstrated 16-Gb/s (4-GBaud × 16-QAM) RoF-based optical heterodyne RF wireless signal delivery at 25.09 GHz with a total of 25.22 km SSMF and 0.5-m to 2-m wireless links achieving EVM and BER well below the standard requirements. The results indicate that monolithically integrated semiconductor InAs/InP QDash BH MML with simple and compact design providing large number of highly correlated optical channels with low noise and high power performance can be a cost-efficient and promising solution for high capacity and high speed RoF-based fronthaul systems of 5G and beyond wireless networks.
Funding
National Research Council Canada (HTSN 201).
Acknowledgments
The authors would like to acknowledge the support of National Challenge Program “High Throughput and Secure Networks (HTSN)” in National Research Council (NRC) Canada.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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